1) Field of the Invention
This invention relates to a cell bridge apparatus and a cell bridging method as well as an information transmission system having a cell bridge apparatus suitable for use with a multimedia transmission system for an ATM network which handles ATM cells as fixed length cells. Here, the term ATM is an abbreviation of Asynchronous Transfer Mode.
2) Description of the Related Art
In recent years, as one of services of a B-ISDN, an integrated video service such as a VOD (Video on Demand) service has been proposed. The integrated video service is provided to allow a subscriber to freely extract and utilize information such as video and audio information stored in a center. Such information is placed into ATM cells and communicated between the center and the subscriber using an ATM technique. In this instance, if a plurality of subscribers require the same information, in order to minimize the traffic on the network to reduce a burden to the center or a repeating unit which serves as a transmission source, it is demanded to deliver only one piece of information to the network and produce a number of copies of it equal to the number of requesting subscribers in each node.
FIG. 15 illustrates a concept of an integrated video service. Referring to FIG. 15, an ATM information transmission system 50 shown includes a center 51 which accepts a request from a customer and distribute video and image data of a movie and so forth, network node interface (NNI) apparatus 52 and 54 having a cell bridging function, an ATM network 53 for distributing cells to a plurality of stations, an ATM network access apparatus 55, subscriber terminals 56-1 to 56-n, and so forth.
The center 51 stores multimedia information of movies and so forth and extracts and distributes, in response to a plurality of requests for the same work from the subscriber terminals 56-i (i=1 to n), only one video data of the work to the ATM network 53, and the ATM network access apparatus 55 copies the video data cell to produce a number of copies equal to the number of requests, thereby minimizing the traffic of the ATM network 53.
The ATM network access apparatus 55 described above is required to apply, to information to be transmitted, a priority degree in accordance with contents of the information and communicate the resulting information. The function of adding a priority degree is realized by bridging, where information D is distributed in a one to multiple fashion (1: multiples), the signal by means of the NNI apparatus 54 or the ATM network access apparatus 55. FIG. 16(a) illustrates an example of bridging by the NNI apparatus 52 or 54, and FIG. 16(b) illustrates an example of bridging by the ATM network access apparatus 55. In both cases, the information D is copied to produce a required number of copies, and simultaneously, new headers H1, H2 and H3 are produced from the header H0 of the ATM cell and outputted together with the copies of the information D to bridge the information D. The bridging illustrated in FIG. 16(a) is called intra-port multicast, and the bridging illustrated in FIG. 16(b) is called multicast.
However, in the technique described above, where it is intended to bridge an ATM cell in such a manner as seen in FIG. 16(a) or 16(b), where the method of copying a cell to produce a number of copies equal to the number of cells to be outputted is employed, a large number of buffers are required, which increases the circuit scale.
Japanese Patent Laid-Open Application No. Heisei 9-83522 discloses a novel technique of an ATM switch which functions as an exchange when data communication by an ATM is performed between terminals or nodes, or more particularly, an ATM switch which is connected to a communication network such as a B-ISDN and can reduce a burden to a transmission source node involved in transmission processing, can eliminate a temporary increase of the traffic of cells in the ATM switch and can prevent an increase of the circuit scale of the ATM switch without increasing the buffer capacity of the ATM switch.
FIG. 17 illustrates operation of a cell bridge apparatus disclosed in Japanese Patent Laid-Open Application No. Heisei 9-83522 mentioned above. Referring to FIG. 17, an ATM network access apparatus 55 or an NNI apparatus 54 (52) shown produces a plurality of headers H1, H2 and H3 based on the header part H0 of a cell C0, copies the data part D1 of the cell D which represents information to produce three copies, reads out data from a buffer based on the headers, composes them to produce new cells C1, C2 and C3, and sends out the new cells C1, C2 and C3. If, in FIG. 17, another cell C4 having a higher priority degree is inputted during outputting of the cell C1 corresponding to the cell C0, then the copying processing for the plurality of headers C2 and C3 for the copies having been produced already must be continued. Consequently, a cell C5 of a higher priority degree originating from the cell C4 is outputted after sending out of the copies is completed.
Referring back to FIG. 15, a multicast method where the cell bridge apparatus shown is used for a node between a point and multiple points is described. As seen from the inside of a frame A in FIG. 15, if a cell C0 which has data D1 and another cell C4 which has data D2 for system control and has a higher priority degree are inputted successively, then after the cell C0 is copied to produce three copies and the thus copied cells C1, C2 and C3 are outputted to the subscriber terminals 56-1, 56-2 and 56-3, respectively, the cell C4 is processed and a resulting corresponding cell C5 is outputted to the subscriber terminal 56-4. In other words, the cell C4 having a higher priority degree is outputted after the multicast is completed.
Accordingly, the technique described above has a subject to be solved in that, when multicast processing is performed by a large amount, transmission of a cell of a higher priority degree is retarded by multicast cells.
It is an object of the present invention to provide a cell bridge apparatus and a cell bridging method as well as an information transmission system having a cell bridge apparatus by which a cell can be outputted in accordance with a priority degree of it even during multicast processing.
In order to attain the object described above, according to an aspect of the present invention, there is provided a cell bridge apparatus, comprising a buffer unit for storing cell data of input cells, a buffer control unit for controlling writing and reading out of the cell data into and from the buffer unit, a cell production control unit for managing multicast information of the cell data read out from the buffer unit by the buffer control unit and producing a cell to be outputted from header information of the cell data, and a cell outputting unit for outputting the cell produced by the cell production control unit and issuing a cell data readout request to the buffer control unit, the buffer unit storing the cell data in accordance with priority degrees of the input cells, the buffer control unit being constructed in such a manner as to read out, when the cell data readout request from the cell outputting unit is received, the input cell data of a high priority degree from among the cell data stored in the buffer unit.
Accordingly, the cell bridge apparatus is advantageous in that it can process a cell having a high priority degree even during multicast processing and moderates the burden to a transmission source by intra-port multicast on the ATM transmission apparatus. The cell bridge apparatus is advantageous also in that, since cells are transmitted in accordance with priority degrees thereof, the transmission efficiency of cells having high priority degrees can be raised.
The buffer unit may include a plurality of buffers provided individually for the different priority degrees of the input cells. This makes the cell bridge apparatus further advantageous in that management of the buffers in accordance with the priority degrees is facilitated.
The buffer control unit may read out, when a readout request from the cell outputting unit is received, one of the input cell data stored in the buffer unit which has the highest priority degree. This makes the cell bridge apparatus further advantageous in that a cell having a high priority degree can be processed during multicast processing and the transmission efficiency of cells having high priority degrees can be raised.
The cell production control unit may include a register for storing an updateable offset value in accordance with a priority degree of the input cell, a router memory for storing header information and multicast information for the cell to be outputted, a router memory control unit for reading out the header information and the multicast information stored in the router memory using address information which is produced based on pointer information obtained from identification information of the cell data read out from the buffer unit by the buffer control unit and the offset value read out from the register based on priority degree information of the input cell which the cell data have, a cell production unit for producing the cell to be outputted from the cell data read out from the buffer unit by the buffer control unit and the header information read out from the router memory by the router memory control unit, and an offset updating unit for updating the offset value stored in the register taking the multicast information read out from the router memory by the router memory control unit into consideration. This makes the cell bridge apparatus further advantageous in that, if an intra-port multicast cell is inputted after processing of a cell having a higher priority degree is completed, then the processing can be resumed from an interrupted portion thereof, and processing in accordance with the priority degrees can be performed.
The cell production control unit may output, when the multicast information read out from the router memory by the router memory control unit is information representing that multicast is unnecessary, a request to the buffer control unit to erase the corresponding cell data, which are being currently processed, stored in the buffer unit. This makes the cell bridge apparatus further advantages in that it can be discriminated whether the same cell data is to be processed only once or repetitively several times, and since the cell data has a different value every time due to the offset value, cells having different routing information can be outputted.
According to another aspect of the present invention, there is provided a cell bridging method for a cell bridge apparatus which includes a buffer unit for storing cell data of input cells, a buffer control unit for controlling writing and reading out of the cell data into and from the buffer unit, a cell production control unit for producing a cell to be outputted from the cell data read out from the buffer unit by the buffer control unit, and a cell outputting unit for outputting the cell produced by the cell production control unit, comprising the steps of storing the cell data of the input cells into the buffer unit in accordance with priority degrees of the input cells, reading out, when the buffer control unit receives a cell data readout request from the cell outputting unit, the input cell data of a high priority degree from among the cell data stored in the buffer unit by the buffer control unit, performing management of multicast information of the cell data read out from the buffer unit by the buffer control unit in accordance with the priority degrees of the cell data and producing a cell to be outputted from header information of the cell data by the cell production control unit, outputting the cell produced by the cell production control unit from the cell outputting unit, and issuing, after the cell is outputted from the cell outputting unit, the cell data readout request to the buffer control unit from the cell outputting unit.
Accordingly, the cell bridging method is advantageous in that, while intra-port multicast is performed, cells can be outputted in accordance with the priority degrees of the same.
The cell bridging method may further comprise a step of reading out, when the cell data readout request is received from the cell outputting unit, by the buffer control unit, the input cell data having the highest priority degree from among the cell data stored in the buffer unit. This makes the cell bridging method further advantageous in that, even during multicast processing, ATM bridging wherein cells can be outputted in accordance with the priority degrees of the same can be performed.
According to a further aspect of the present invention, there is provided an information transmission system, comprising a center unit for storing information, a plurality of terminals which utilize the information from the center unit, and a cell bridge apparatus for performing multicasting for the information transmitted over a network, which handles fixed length cells, from the center unit to transmit the information to the plurality of terminals, the cell bridge apparatus including a buffer unit for storing cell data of input cells, a buffer control unit for controlling writing and reading out of the cell data into and from the buffer unit, a cell production control unit for managing multicast information of the cell data read out from the buffer unit by the buffer control unit and producing a cell to be outputted from header information of the cell data, and a cell outputting unit for outputting the cell produced by the cell production control unit and issuing a cell data readout request to the buffer control unit, the buffer unit storing the cell data in accordance with priority degrees of the input cells, the buffer control unit being constructed in such a manner as to read out, when the cell data readout request from the cell outputting unit is received, the input cell data of a high priority degree from among the cell data stored in the buffer unit.
Accordingly, the information transmission system is advantageous in that the burden to a transmission source on the ATM network can be moderated and the transmission rate of data having high priority degrees can be raised.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.